Theta-gamma coupling in the entorhinal-hippocampal system

•Theta-gamma coupling supports memory processes in the entorhinal-hippocampal network.•Theta-modulated slow gamma may promote memory retrieval.•Theta-modulated fast gamma may facilitate memory encoding.•Theta-nested gamma reflects gamma rhythmic inhibition and theta rhythmic excitation.

For decades, theta rhythms (∼5-10 Hz) have been thought to play a critical role in memory processing in the entorhinal-hippocampal network. However, recent evidence suggests that successful memory performance also requires coupling of ∼30-100 Hz gamma rhythms to particular phases of the theta cycle. Recent insights imply ways in which theta-gamma coupling may facilitate transfer of information throughout the entorhinal-hippocampal network. Activating gamma-modulated cell assemblies at a particular theta phase may allow the network to produce a more powerful output by ensuring that distributed cells fire closely in time. I hypothesize that such a mechanism would serve to facilitate either memory encoding or memory retrieval, depending on which type of gamma rhythms are recruited.